2 answers

QRadar installations are based off of the capability of the hardware and there is no minimum version or software check that would prevent you from running QRadar 7.2.0 on an M5 appliance. It is not recommended to use a version that old, but I believe that M5s ship with a version of QRadar 7.2.8 from the factory. As IBM started shipping M5s in January, 2017 or December, 2016. This was before QRadar 7.3.0 was available.

If you were to order an M5 today, it would likely ship with either QRadar 7.2.8 or QRadar 7.3.0. I would have to talk to someone in manufacturing or sales to get a more certain answer as to if you wanted 7.2.8 over 7.3.0. It would depend on existing inventory I would expect.